Fix restorePrivateDnsSetting with null hostnames

When private DNS setting was set to opportunistic (mode) and null
(hostname), CtsNetUtils would not restore it.

Make sure that private DNS settings are restored after every test. Also
fail if restore is called without having saved any setting beforehand.

Bug: 190465704
Test: atest CtsNetTestCases
Change-Id: Ic5d8d8b729469e0eef89a0b53f166e604264c1ee
1 file changed